1. Field of the Invention
The present invention relates to an encryptor, a decryptor, a cryptographic processor, and a computer system, more specifically, to an encryptor for encrypting plain text, a decryptor for decrypting cypher text, and a cryptographic processor for cryptographic service, and a computer system for the purpose of protecting secret data against intrusion.
2. Description of the Prior Art
Recently, as computer networks are expanding rapidly, techniques for protecting critical digital information have been remarked. One of such techniques is to store digital information (referred to as secret data hereinbelow) to be secured into a storage device after encryption.
Sufficient attention should be paid for safety operation of encryption technique, especially in the dealing of secret information such as encryption keys and decryption keys. In general, for an encryption technique within an communication devices, such secret information is stored in a non-volatile memory. Such a non-volatile memory is enclosed in a molded resin so as to protect against intrusion. This may allow secret information not to be leaked if sufficient access control is performed.
However, if some highly value-added data is encrypted and if its secret information (decryption key) is protected with such a level of protection, the secret information may be stolen. A molded resin may be removed by melting it, then it may be possible that any secret information may be stolen by probing charge information in a memory cell. At present, smart cards have been applied in the field of electric money and electric commerce. In these fields, the problem described above becomes so serious that it cannot be ignored.
In order to prevent piracy, there is a method in which secret information is stored in a RAM (Random Access Memory), a kind of volatile memory (see, Japanese Published Examined Application No. S61-61740 entitled "communication secret apparatus"). RAM is supplied with power through a micro-switch. If the box incorporating the RAM is pulled out from its attachment for the purpose of stealing secret information, the micro-switch opens to cut off the power supply. When the power is interrupted, the secret information stored in the RAM evaporates to ensure the security.
However, this method has a disadvantage that the data is not evaporated by cutting off the power supply if the box incorporating the RAM is held in a cryogenic temperature environment.
There is also another method of making a special box for preventing intrusion (see, Japanese Published Examined Application No. S63-78250 entitled "a data security device for protecting stored data"). This special box is formed of a top plate, a bottom plate, and four side plates including curved conductive wires respectively. Two conductive wires compose parallel conductive lines. A detection circuit is formed to generate a reset signal if there is an intrusion by creating a short circuit or earth connection of the conductive lines. The secret information stored in a memory within this box is erased when a reset signal is generated. The plates forming the box are made of ceramics, which protect against chemical attack. In addition, a temperature sensor is provided within the box in order to protect from an attack attempting to knock out the detection circuit by freezing.
There has been proposed a method for improving the sensitivity against intrusion into the box, in which a barrier protects an electric assembly from mechanical or chemical attack (see, U.S. Pat. No. 5,027,397 and 5,159,629 entitled "Data protection by detection of intrusion into electric assembles"). The intrusion barrier includes a screen material surrounding the electronic assembly, on which screen conductive lines are formed, and conductors connected to power supply means and to signal detector means. These conductive lines are formed of conductive particles of material dispersed in a solidified matrix. These conductive lines are very finely patterned so as to change resistance when a mechanical or chemical attack is made to the intrusion barrier.
By applying such a intrusion barrier to the electronic assembly, if a chemical or mechanical attack is attempted, some conductive particles lose their mechanical integrity so that the intrusion detector detects the variance of resistance of conductor circuit and thereby erase the secret information in the volatile memory.